1. Field of the Invention
Synchronous machines have long been known in many different forms. In operation as a generator and also in operation as a motor, in the situation of a short-circuit, for example a short-circuit at connecting terminals or also within the stator winding, such synchronous machines produce very high short-circuit torques. In that respect, those very high short-circuit torque peaks can reach values of up to six to eight times the rated torque of the synchronous machine. Therefore, consideration must also be given to the very rare short-circuit situation and the concomitant occurrence of a very high short-circuit torque, in regard to the mechanical construction of the synchronous machine.
2. Description of the Related Art
Synchronous machines of the above-described kind have already long been used in wind power installations from the company Enercon. In those types of wind power installations, the synchronous machines are in the form of ring generators, wherein the generator rotor rotates within the generator stator and the generator rotor is mounted by a flange mounting directly to the rotor of the wind power installation. In the case of a generator rotor which is directly mounted by a flange mounting to a drive machine, very high short-circuit torques occur in a short-circuit situation and the constructions are linked to very high cost levels and a very high level of maintenance, in order to avoid major damage.
Therefore, in a synchronous machine as is used for example in the wind power installation of type E-40 from Enercon, a mechanical safety device in the form of a shearing pin coupling was developed and used therein. In that case the so-called stator carrier star or spider (carrier) which carries the stator (stationary member of the generator) is connected by way of shearing pins to the axle journal or stub axle which is also stationary. In the event of a generator short-circuit, the pins shear and permit the stator also to rotate about the axle journal. In that way the transmitted torque is limited to a maximum of four times the rated torque and the drive train is guaranteed to be safeguarded in a generator short-circuit situation.
DE 197 29 034 discloses a wind power installation with a synchronous generator which has a generator stator with a stator winding and a generator rotor which is movable relative to the stator. In one embodiment the stator there has a 6-phase stator winding. The 6 phases of the synchronous generator are connected to a common rectifier circuit.
WO 88/07782 discloses a wind power installation with an electrical generator having a rotor and a stator. The stator there has a plurality of turns which can be connected together in different ways by suitable switching means in order to produce a desired output signal.
DE 40 32 492 discloses an electrical machine for power converter operation comprising a multi-phase stator winding which can be switched over, that can also be used in a wind power installation. The stator winding in that case is subdivided into similar, respectively m-phase winding subsystems which are galvanically separated and fixedly connected in a star or polygon configuration. Special switching elements are provided for switching over the winding.
The object of the invention is to simplify the synchronous machine in regard to its structure and to avoid the above-mentioned disadvantages.
In accordance with the invention that object is attained by a synchronous machine having the feature set forth in claim 1. Advantageous developments are described in the further claims.
The invention is based on the realisation that particular safety elements such for example a shearing pin coupling are not necessary if the maximum short-circuit torque is limited to a fraction of that which is hitherto usual. Preferably, in the synchronous machine according to the invention, the short-circuit torque is always less than double the torque.
For limiting the short-circuit torque the rotor has at least two independent three-phase windings which are electrically and/or spatially isolated from each other. That provides that the power of the generator is divided to two different three-phase systems. With two independent three-phase systems, each system involves only 50% of the rated power. Those systems are displaced through an angle of 30xc2x0. This means that both three-phase systems are electrically and mechanically (spatially) isolated. In that way however the reactance Xd is also approximately doubled and thus the short-circuit current is halved. This affords the particular advantage that, in the event of a short-circuit in a system, only half the short-circuit power can occur. It is thus possible to have a reduction in the maximum short-circuit moment (short-circuiting of two phases, for example U1 and V1) by 50% relative to the hitherto usual single-system arrangement.
Another measure for reducing the short-circuit torque is to eliminate a damper cage, especially as the reactances Xdxe2x80x3 and Xdxe2x80x2 determine the dynamic short-circuit current configuration. The maximum short-circuit torque can be reduced by about 30% by virtue of the omission of the damper cage, that is to say by virtue of the use of a salient-pole machine without damper winding.
As a further measure for reducing the torque involved, it is proposed that the pole heads of the rotor are of an approximately arrow-shaped configuration. In the case of a short-circuit in one of the two three-phase systems the magnetic flux in the exciter pole can then very rapidly deflect in the direction of rotation. That affords soft dynamic decoupling of the exciter flux with the stator flux within a groove. Per pole width, the short-circuit current then flows only in two of a total of six grooves. That dynamic decoupling of the exciter flux still further reduces the short-circuit torque.